Implement FromStr for zonemd Scheme and Algorithm

[mozzieongit]

While writing the dnst signzone zonemd argument parsing, I noticed that zonemd::Scheme and zonemd::Algorithm implement neither FromStr nor a from_mnemonic function (as with the enums created with the iana int_enum macro).
I was unsure which one should be used, but settled on FromStr with both number and mnemonic, as it allows an easy use of str::parse.